The subject matter described herein relates generally to methods and systems for resonance dampening, and more particularly, to methods and systems for resonance dampening in wind turbines.
Generally, a wind turbine includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some wind turbine configurations include double-fed induction generators (DFIGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
Known wind turbines have a plurality of mechanical and electrical components. Each electrical and/or mechanical component may have independent or different operating limitations, such as current, voltage, power, and/or temperature limits, than other components. Moreover, known wind turbines typically are designed and/or assembled with predefined rated power limits. To operate within such rated power limits, the electrical and/or mechanical components may be operated with large margins for the operating limitations. Such operation may result in inefficient wind turbine operation, and a power generation capability of the wind turbine may be underutilized.
Modern wind turbines require active damping of mechanical resonances/vibrations in order to reduce mechanical loads, for instance at the drive train and blades, or to avoid instability. U.S. Pat. No. 7,501,798 discloses a method for the active damping of a drive train in a wind energy plant, wherein a correction moment for a generator control is determined. U.S. Pat. No. 7,309,930 discloses a vibration damper which provides a variable signal to control torque produced by a generator of the wind turbine system. The variable torque control signal is based on generator speed and has a first local peak value based on a resonant frequency of an oscillation of a tower of the wind turbine.
Modern wind turbines are deployed in a large variety of electrical grid environments. The effectiveness of resonance damping is dependent on the setup and conditions of the electrical grid the turbine is connected to. If the grid conditions are not known, or have some variability with time, it is challenging to design a resonance damper which will work under each grid condition which may occur during operation. For at least some conditions not accounted for in the original design of the damping system, a conventional resonance damper may lead to increased mechanical loads and can reduce the lifetime of wind turbine components.
One prior attempt of solving this problem was by individual retuning of the resonance damper based on local grid conditions. This, however, is an expensive approach as it requires each location to be treated individually. Further, grid conditions do not only vary between sites, but can also change over time, and the exact conditions might not even be known. This can generally not be handled with retuning.
In view of the above, it is desirable to have a resonance dampening method and system for wind turbines which avoids the cited disadvantages.